Update to Faster math conversions

[Doprez]

I was looking through Mono to see if you all were having the same problems of the existing System.Numerics library built into .NET or if you were still using your own math classes.

I noticed the conversion methods were creating new values instead of using the Unsafe class which has been very helpful for our Bepu to Stride conversions that were a bottleneck before finding this option.

This PR changes the built in conversions to use Unsafe.As to convert instead of creating a new value.

The one thing I dont understand is why the implicit conversions for Matrix are as fast as the Unsafe.As but the implicit for Vector3 and Quaternion are what I would expect from using the new keyword.

This is the example where I forced Matrix to use the new keyword the rest are using the existing implicit conversion:

And this is the example of using the Matrix implcit conversion:

here is the gist files of the benchmarks I ran https://gist.github.com/Doprez/04d61e03915a9527a482cddbaa826544

[Mindfulplays]

Wondering if there needs to be copious doc in Matrix/Vector* structs etc to always mirror Numerics layout? These are not things that will ever change but mostly as a way to ensure across platforms/byte-order etc it should hold true. (Tests?)

Also, it's kind of implicit but ref <struct> doesn't cause an alloc (cool) but may need a doc comment explaining the gymnastics happening here to help the compiler use the Numerics without any allocs.. i.e. this

[Doprez]

are you talking about docs to inform developers using the xml comments? or are you talking about the layout attribute to ensure the struct matches the numerics attribute explicitly? I may be missing something so please let me know but both are row major matrices so I believe the memory layout should match. Tests would be a great idea though, just to be sure.

Instead of ToMonoGame(this System.Numerics.Matrix4x4 matrix) it should be ToMonoGame(this ref System.Numerics.Matrix4x4 matrix) to ensure the clarity of whats expected right? I can add a better description to the xml doc as well if it feels needed.

[Mindfulplays]

are you talking about docs to inform developers using the xml comments? or are you talking about the layout attribute to ensure the struct matches the numerics attribute explicitly? I may be missing something so please let me know but both are row major matrices so I believe the memory layout should match. Tests would be a great idea though, just to be sure.

I meant both: xml comment and a layout attribute. especially people may not be aware that there is link between the two. (note: am not a maintainer, so this is mostly a drive-by :) )

Instead of ToMonoGame(this System.Numerics.Matrix4x4 matrix) it should be ToMonoGame(this ref System.Numerics.Matrix4x4 matrix) to ensure the clarity of whats expected right? I can add a better description to the xml doc as well if it feels needed.

Here, I meant that you are also avoiding extra allocs (because of ref <struct>) which is kinda nice - but to me it was not obvious at first glance and hence I had to double check dotnet docs :) simply adding that to the xml comment will be nice...

[mrhelmut]

This will need to wait for the console runtime to be fully switched to NativeAOT and will also need to be tested on consoles, I have some doubts about System.Runtime.CompilerServices in that context.